Serotonin antagonists do not attenuate activity-induced phase shifts of circadian rhythms in the Syrian hamster

A variety of observations from several rodent species suggest that a seroto nin (5-HT) input to the suprachiasmatic nucleus (SCN) circadian pacemaker m ay play a role in resetting or entrainment of circadian rhythms by non-phot ic stimuli such as scheduled wheel running. If 5-HT activity within the SCN is necessary for activity-induced phase shifting, then it should be possib le to block or attenuate these phase shifts by reducing 5-HT release or by blocking post-synaptic 5-HT receptors. Animals received one of four seroton ergic drugs and were then locked in a novel wheel for 3 h during the mid-re st phase, when novelty-induced activity produces maximal phase advance shif ts. Drugs tested at several doses were metergoline (5-HT1/2 antagonist; i.p .), (+)-WAY100135 (5-HT1A postsynaptic antagonist, which may also reduce 5- HT release by an agonist effect at 5-HT1A, raphe autoreceptors; i.p.), NAN- 190 (5-HT1A postsynaptic antagonist, which also reduces 5-HT release via an agonist effect at 5-HT1A raphe autoreceptors; i.p.) and ritanserin (5-HT2/ 7 antagonist; i.p. and i.c.v.). Mean and maximal phase shifts to running in novel wheels were not significantly affected by any drug at any dose. Thes e results do not support a hypothesis that 5-HT release or activity at 5HT1 , 2 and 7 receptors in the SCN is necessary for the production of activity- induced phase shifts in hamsters. (C) 1998 Published by Elsevier Science B. V. All rights reserved.